Although largely unappreciated, coatings have made possible the tremendous growth in the use of aluminum cans that has occurred in recent years. In this role, coatings not only serve an esthetic purpose in providing a highly attractive, clear, glossy consumer product, but they must possess many functional properties as well. For example, the coatings must protect the underlying printed surface from abrasion and from scratching during distribution and handling. Successful coatings must possess excellent adhesion, toughness, lubricity and hardness. The coating must especially possess excellent abrasion resistance for protection against abrasive failure of the container and loss of its contents. Coatings should also possess sufficient hydrolytic stability and adhesion to survive sterilization for those situations in which the container will be sterilized after coating. In addition, the coating materials in the uncured state should have good flow and leveling properties and be capable of high speed application and cure. Further, the components of the coating should have a low level of oral, skin, and eye toxicity and should not contribute to or change the taste of any food contents in the container. Lastly, the cost of a coating material designed for one-time use must be inherently low.
With one exception, discussed below, virtually all major manufacturers of aluminum cans coat the cans with thermally cured polymer coatings. In a thermal cure process, the polymer, in solution, is applied to the container and the container is transferred to a long pin chain which travels through a thermal dryer. The conventional thermal drying oven used to accommodate speeds of 1500 cans per minute (cpm) would be approximately 50 feet long, 25 feet high and 8 feet wide. The pin chain required could be 400 feet long since the ink and coating chemistry require a peak metal temperature of 350 degrees F to initiate cross-linking. Conventional thermally cured resin mixtures contain 30 to 40% solvents, which, under the curing conditions, are volatilized to leave behind a solid film. The thermal process has a number of unattractive features: (1) it generates significant VOC (volatile organic compound) emissions which will soon come under more stringent regulation; (2) starting up and shutting down for repairs are time-consuming because a very large mass must be heated and cooled; and (3) thermal curing ovens are several times more expensive to buy and, because they use a lot of energy, several times more expensive to operate than UV curing chambers.
The alternative to thermal curing is UV curing. In the UV process, cans are coated in the same manner as in the thermally cured system, but instead of a long pin chain taking them into a thermal oven, the cans travel on a short section of pin chain, are picked up by a vacuum belt conveyor, and then are conveyed inside the UV chamber. Once inside the chamber, the non-rotating cans are cured using a bank of UV lamps. A 1500 cpm UV curing oven is about 15 feet long, 6 feet high and 3 feet wide. Approximately 40 feet of vacuum belt are used to carry the cans through the chamber. Curing temperatures for the UV chamber are approximately 120.degree. F. Since UV lamps are instant on/off, the chamber is at room temperature in a matter of minutes from shutdown. In addition, the UV chamber has simpler controls because there are no gas valves, heat dump system or large blowers necessary to remove oven exhaust. UV chambers have virtually no moving mechanical parts and all the electrical components are modular (a faulty lamp or power supply can be easily replaced). The UV coating process generates much less VOC emission because there is no solvent.
With all the apparent advantages of a UV-curable coating system, the question arises, "why does it constitute only a small fraction of the present market?" The process is fine: the problem is with the resins. Primarily because of cost factors, the only UV-curable resins presently in use are acrylates. They suffer from several drawbacks: (1) they have poor adhesion to metal; (2) the monomers are potent irritants and sensitizing agents; (3) they are polymerized by a free-radical mechanism, and the polymerization is inhibited by oxygen (air) and (4) their abrasion resistance is only marginally acceptable.
There is thus a need for an inexpensive, UV-curable coating resin that has good adhesion, is non-toxic and non-sensitizing, releases no VOCs on curing, is rapidly cured, and which results in a coating that is attractive and durable.